0-RTT
Zero Round-Trip Time
A TLS 1.3 shortcut that lets a browser send its request in the very first packet to a server it has spoken to before, saving a full round trip.
In URL
Glossary
Every acronym that appears anywhere in the guides, with what it stands for and what it actually means. Search it, filter it by topic, or link straight to a term.
141 terms
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141 terms
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#
2G / 3G / 4G / 5G
Second to fifth generation mobile
The generations of mobile network. Each one is a different radio standard, not just a faster version of the last; 5G is the current one.
In 5G
802.11
IEEE 802.11
The family of standards that defines Wi-Fi. The letters after it mark the generation — 802.11n, ac, ax — which the Wi-Fi Alliance now renames Wi-Fi 4, 5 and 6.
In Wi-Fi
802.1X
IEEE 802.1X
Port-based network access control: the device must prove who it is to an authentication server before the network will carry any of its traffic. The basis of enterprise Wi-Fi logins.
In Wi-Fi
A
AAAA
Quad-A record
The DNS record type that holds an IPv6 address, the way an A record holds an IPv4 one.
In URL
ACK
Acknowledgement
A short message saying "I received that". Wi-Fi acknowledges every frame; TCP acknowledges every run of bytes. No acknowledgement means the sender tries again.
A-MPDU
Aggregated MAC Protocol Data Unit
Many Wi-Fi frames bundled into one transmission so the airtime overhead is paid once instead of dozens of times.
In Wi-Fi
ANonce
Authenticator Nonce
The random number the access point contributes to the four-way handshake. A nonce is a number used once, so the same password never produces the same key twice.
In Wi-Fi
AP
Access Point
The Wi-Fi radio that devices connect to — in a home it is built into the router, in an office it is a separate ceiling unit.
In Wi-Fi
ARP
Address Resolution Protocol
How a device finds the MAC address that goes with an IP address on the local network. It shouts "who has 192.168.1.1?" and waits for the owner to answer.
In Wi-Fi
B
BIOS
Basic Input/Output System
The original PC firmware, limited to a 512-byte boot sector and about a megabyte of space. Replaced by UEFI, though people still say "the BIOS" for any firmware settings screen.
In Startup
BPSK
Binary Phase-Shift Keying
The simplest and most robust Wi-Fi modulation: one bit per symbol. Slow, but it survives a terrible signal.
In Wi-Fi
BSS
Basic Service Set
One access point and the devices attached to it — a single cell of a Wi-Fi network.
In Wi-Fi
BSSID
Basic Service Set Identifier
The MAC address of one particular access point radio. Several access points share an SSID, but each has its own BSSID, which is how roaming can be observed.
In Wi-Fi
C
CA
Certificate Authority
An organisation your browser already trusts, which signs website certificates. The padlock means a CA vouched for the name on the certificate, nothing more.
In URL
CCMP
Counter Mode CBC-MAC Protocol
The AES-based encryption WPA2 uses to protect Wi-Fi frames.
In Wi-Fi
CDN
Content Delivery Network
A network of servers spread around the world that keep copies of a site's files, so the response comes from a nearby city rather than the origin server.
In URL
CNAME
Canonical Name
A DNS record that says "this name is really that name" — an alias that the resolver then has to look up in turn.
In URL
CPU
Central Processing Unit
The processor. At power-on it starts executing at a fixed address called the reset vector, with no memory and no operating system yet.
In Startup
CSS
Cascading Style Sheets
The language that describes how a web page looks. The browser must fetch and parse it before it can paint anything.
In URL
CSSOM
CSS Object Model
The browser's parsed, in-memory version of the stylesheets, combined with the DOM to work out what every element should look like.
In URL
CTS
Clear To Send
The reply to an RTS. The pair reserves the air for one device, which stops two stations that cannot hear each other from talking over one another.
In Wi-Fi
D
dB
Decibel
A ratio on a logarithmic scale. Every 3 dB is double or half the power, and every 10 dB is ten times — which is why small dB numbers describe enormous differences.
dBm
Decibels relative to one milliwatt
The usual unit for Wi-Fi signal strength. It is negative in practice: -50 dBm is excellent, -70 dBm is workable, -80 dBm is trouble.
In Wi-Fi
dBW
Decibels relative to one watt
The unit used for satellite footprint maps. A higher dBW figure over your area means a smaller dish will do.
In Satellite
DDR5
Double Data Rate 5
The current generation of system memory. The firmware has to train it — tuning timings until the chips read back correctly — before it can be used at all.
In Startup
DFS
Dynamic Frequency Selection
The rule that makes Wi-Fi vacate certain 5 GHz channels when it detects radar, because those channels are shared with weather and military systems.
In Wi-Fi
DHCP
Dynamic Host Configuration Protocol
How a device that has just joined a network is handed an IP address, a gateway and a DNS server, instead of being configured by hand.
In Wi-Fi
DIFS
DCF Interframe Space
The longer gap a Wi-Fi device must wait before it may start transmitting. Shorter waits get priority, which is how acknowledgements jump the queue.
In Wi-Fi
DKIM
DomainKeys Identified Mail
A DNS-published signature that lets a receiving mail server check a message really came from the domain it claims.
In URL
DNS
Domain Name System
The internet's directory: it turns a name people can remember into the IP address a machine can route to. Nothing else can start until it answers.
DOCTYPE
Document Type Declaration
The first line of an HTML file. Its only real job now is to keep the browser in standards mode rather than an old compatibility mode.
In URL
DOM
Document Object Model
The browser's tree-shaped, in-memory model of the HTML. Scripts change the page by changing the DOM, not the original file.
In URL
DVB-S2
Digital Video Broadcasting — Satellite, 2nd generation
The standard that defines how a satellite TV signal is modulated and error-protected. Its efficiency is why a modern transponder carries far more channels than it used to.
In Satellite
E
EAP
Extensible Authentication Protocol
The framework 802.1X uses to carry a login — a username and password, or a certificate — between the device and the authentication server.
In Wi-Fi
ECM
Entitlement Control Message
The constantly changing decryption key for a scrambled channel, encrypted and broadcast to everyone. Only a card that is entitled can unwrap it.
In Satellite
EFI
Extensible Firmware Interface
The interface UEFI implements. In practice you meet it as the \EFI folder on the boot partition, holding one bootloader file per operating system.
In Startup
EGNOS
European Geostationary Navigation Overlay Service
A European augmentation system that broadcasts correction data, sharpening GPS from metres to roughly a metre.
In GPS
EIRP
Effective Isotropic Radiated Power
How much signal power actually arrives at a given point on the ground. It decides the dish size you need, and it is fixed when the satellite is built.
In Satellite
EMM
Entitlement Management Message
A message addressed to one specific smart card, telling it which channels the subscription covers. Turning a package on or off is an EMM.
In Satellite
EPG
Electronic Programme Guide
The on-screen schedule. It arrives as just another stream of data packets alongside the picture and sound.
In Satellite
eSIM
Embedded SIM
A SIM soldered into the phone and provisioned over the air, rather than a plastic card you swap. Same role, no tray.
In 5G
ESP
EFI System Partition
A small FAT partition the firmware can read without any operating system drivers. It holds the bootloaders.
In Startup
F
FAT
File Allocation Table
A deliberately simple filesystem. It is used for the EFI System Partition precisely because firmware can implement it in very little code.
In Startup
FCS
Frame Check Sequence
A checksum at the end of a Wi-Fi frame. If it does not match, the frame is discarded and never acknowledged, so the sender retransmits.
In Wi-Fi
FEC
Forward Error Correction
Deliberate redundancy added to a transmission so the receiver can repair errors without asking for anything to be resent — essential when the sender is 36,000 km away.
In Satellite
G
GCMP
Galois/Counter Mode Protocol
The encryption WPA3 and faster Wi-Fi use in place of CCMP. Stronger, and cheaper to run at high data rates.
In Wi-Fi
GDM
GNOME Display Manager
The Linux program that draws the login screen and starts your desktop session once you have authenticated.
In Startup
GET
HTTP GET
The request method meaning "give me this". It should never change anything on the server, which is why it is safe to repeat, cache and bookmark.
In URL
GHz
Gigahertz
A billion cycles per second. Wi-Fi uses 2.4, 5 and 6 GHz bands; satellite TV arrives in the Ku band around 10–12 GHz.
GLONASS
Globalnaya Navigatsionnaya Sputnikovaya Sistema
Russia's satellite navigation system. Most phones use it alongside GPS, which is why they can see far more satellites than GPS alone provides.
In GPS
GNSS
Global Navigation Satellite System
The umbrella term for all of them — GPS, GLONASS, Galileo and BeiDou. "GPS" in everyday speech usually means GNSS.
In GPS
GPS
Global Positioning System
The American satellite navigation system: about thirty-one satellites broadcasting the time, from which a receiver works out where it is. It only listens — it never transmits.
In GPS
GPU
Graphics Processing Unit
The chip that does the actual drawing. The browser hands it the finished layers to composite onto the screen.
In URL
GRUB
GRand Unified Bootloader
The bootloader most Linux systems use. It is what shows the menu of kernels and other operating systems.
In Startup
GTK
Group Temporal Key
The single shared key that encrypts broadcast and multicast Wi-Fi traffic, handed out during the four-way handshake.
In Wi-Fi
H
H.264
Advanced Video Coding
The video compression standard behind most HD broadcasting. Widely supported, and less efficient than HEVC.
In Satellite
HD
High Definition
Broadcast pictures at 1280x720 or 1920x1080, as opposed to the older standard-definition formats.
In Satellite
HDMI
High-Definition Multimedia Interface
The single cable that carries picture, sound and control signals from the set-top box to the television.
In Satellite
HEAD
HTTP HEAD
A request for the headers only, with no body — used to check whether something has changed without downloading it.
In URL
HEVC
High Efficiency Video Coding
The successor to H.264, also called H.265. Roughly half the bitrate for the same picture, which is what makes 4K broadcasting practical.
In Satellite
HSTS
HTTP Strict Transport Security
A header telling the browser to only ever use HTTPS for this site from now on, so a plaintext request is never sent — not even the first redirect.
In URL
HTML
HyperText Markup Language
The language web pages are written in. The browser parses it into the DOM as it arrives, rather than waiting for the whole file.
In URL
HTTP
HyperText Transfer Protocol
The request-and-response protocol the web runs on: a method, a path, some headers, and a body coming back.
HTTP/2
HTTP version 2
A binary version of HTTP that multiplexes many requests over one connection, removing the queueing that made HTTP/1.1 slow.
In URL
HTTP/3
HTTP version 3
HTTP carried over QUIC and UDP instead of TCP, so one lost packet no longer stalls every other request on the connection.
In URL
HTTPS
HTTP Secure
HTTP inside a TLS tunnel. The protocol is unchanged; everything is simply encrypted and the server's identity is checked first.
In URL
I
IC
Integrated Circuit
A chip. A laptop's power management IC brings up a dozen voltage rails in a precise order before the processor may start.
In Startup
IP
Internet Protocol
The addressing and routing scheme the internet is built on. Every packet carries a source and destination IP address and nothing else about the conversation.
IPv4
Internet Protocol version 4
The original addressing scheme, written as four numbers such as 93.184.216.34. There are not enough of them, which is why NAT exists.
In URL
IPv6
Internet Protocol version 6
The much larger address space, written in hexadecimal groups. Enough addresses that every device can have its own.
In URL
ISP
Internet Service Provider
The company that carries your traffic to the rest of the internet, and usually runs the DNS resolver your router is handed by default.
In URL
K
L
LAN
Local Area Network
The network inside one building or home, as opposed to the internet beyond the router.
LNB
Low-Noise Block downconverter
The unit on the dish arm. It amplifies the very weak incoming signal and shifts it down to a frequency a coaxial cable can carry indoors.
In Satellite
LSASS
Local Security Authority Subsystem Service
The Windows process that checks your password and issues the tokens that decide what you are allowed to do.
In Startup
LVM
Logical Volume Manager
A layer between partitions and filesystems on Linux, letting volumes be resized and spanned across disks.
In Startup
M
MAC address
Media Access Control address
The hardware address of a network interface, written as six pairs of hexadecimal digits. Unique per radio, and the reason phones now randomise it.
In Wi-Fi
Mbps
Megabits per second
A million bits per second. Note bits, not bytes — divide by eight for megabytes.
MCS
Modulation and Coding Scheme
The index describing how densely data is packed into the radio signal. A clean link supports a high MCS; a noisy one forces the rate down.
In Wi-Fi
MHz
Megahertz
A million cycles per second. Wi-Fi channels are 20 to 160 MHz wide, and the width is one of the three things that sets the speed.
MIC
Message Integrity Check
Proof attached to a handshake message showing the sender holds the right key, and that nothing was altered in transit.
In Wi-Fi
MIMO
Multiple Input, Multiple Output
Using several antennas at each end to send independent streams of data at the same time on the same frequency.
MPDU
MAC Protocol Data Unit
One Wi-Fi frame as it goes over the air, headers and all.
In Wi-Fi
MU-MIMO
Multi-User MIMO
MIMO aimed at several devices at once, so one transmission can serve more than one client instead of taking turns.
In Wi-Fi
MX
Mail Exchange
The DNS record that says which server handles email for a domain.
In URL
N
NAT
Network Address Translation
The router's trick of making every device behind it share one public IP address, rewriting addresses and ports as traffic passes through.
In Wi-Fi
NR
New Radio
The radio standard that makes 5G 5G, as distinct from the core network behind it.
In 5G
NS
Name Server
The DNS record naming which servers are authoritative for a domain — the ones with the real answers.
In URL
NXDOMAIN
Non-Existent Domain
The DNS answer meaning the name simply does not exist, as opposed to existing with no address.
In URL
O
OFDMA
Orthogonal Frequency-Division Multiple Access
Splitting one Wi-Fi channel into subcarriers that can be handed to different devices at the same instant, instead of making each wait its turn.
In Wi-Fi
OPTIONS
HTTP OPTIONS
A request asking what a server will allow. Browsers send it automatically as the preflight check before certain cross-origin requests.
In URL
P
PAM
Pluggable Authentication Modules
The Linux framework that decides how a login is checked — password, fingerprint, smart card — without every program implementing it itself.
In Startup
PBKDF2
Password-Based Key Derivation Function 2
The deliberately slow calculation that turns a Wi-Fi password into the PMK. Slowness is the point: it makes guessing expensive.
In Wi-Fi
PCR
Platform Configuration Register
A register in the TPM holding a running hash of everything loaded so far. It can be extended but never rewritten, which is what makes measured boot trustworthy.
In Startup
PHY
Physical layer
The radio itself. The PHY rate is the headline speed negotiated between two radios — always well above the throughput you actually get.
In Wi-Fi
PID
Packet Identifier
The number on every satellite transport packet saying which stream it belongs to — one PID for picture, another for sound, another for subtitles.
In Satellite
PID
Process ID
The number the operating system gives each running process. PID 1 is the first one started, and the ancestor of everything else.
In Startup
PMK
Pairwise Master Key
The key derived from the Wi-Fi password and network name. It never crosses the air — both ends calculate it independently.
In Wi-Fi
POST
Power-On Self-Test
The firmware's check that the essential hardware is present and working before it tries to boot anything.
In Startup
POST
HTTP POST
The request method meaning "here is something, do it". Repeating it repeats the action — hence the warning about pressing back after submitting a form.
In URL
PTK
Pairwise Transient Key
The session key protecting traffic between one device and one access point, built during the four-way handshake and thrown away afterwards.
In Wi-Fi
PXE
Preboot Execution Environment
Booting from the network: the firmware fetches a bootloader over the LAN instead of from a local disk. Pronounced "pixie".
In Startup
Q
QAM
Quadrature Amplitude Modulation
Encoding data as distinct combinations of amplitude and phase. 4096-QAM packs twelve bits into one symbol, but needs a near-perfect signal to tell the positions apart.
In Wi-Fi
QUIC
A transport protocol built on UDP that carries HTTP/3. Not an acronym any more — the name simply stuck.
In URL
R
RADIUS
Remote Authentication Dial-In User Service
The server that enterprise Wi-Fi asks whether a username and password are valid. The name is a relic of dial-up.
In Wi-Fi
RAID
Redundant Array of Independent Disks
Combining several drives so they behave as one, for speed, for redundancy, or both.
In Startup
RAM
Random Access Memory
The machine's working memory. At power-on there is none usable, so the processor runs from cache configured as RAM until the memory is trained.
In Startup
ROM
Read-Only Memory
Memory that cannot be written in normal operation. The first instructions after reset come from something of this kind.
In Startup
RSSI
Received Signal Strength Indicator
The measure of how strong a received Wi-Fi signal is, usually reported in dBm. Strength alone does not mean a good link — noise matters just as much.
In Wi-Fi
RTS
Request To Send
A short frame asking to reserve the air before a long transmission, answered by a CTS.
In Wi-Fi
RTT
Round-Trip Time
How long a packet takes to get there and back. Every extra round trip in a handshake costs one of these, which is why distance is felt as latency.
In URL
S
SAE
Simultaneous Authentication of Equals
The WPA3 handshake that replaced WPA2's. Capturing it gives an attacker nothing to take away and guess at offline.
In Wi-Fi
SDDM
Simple Desktop Display Manager
The login screen used by KDE and several other Linux desktops.
In Startup
SIFS
Short Interframe Space
The shortest gap between Wi-Fi transmissions. Acknowledgements use it, so they always get in ahead of any new traffic.
In Wi-Fi
SIM
Subscriber Identity Module
The card holding the secret key that proves to the network which subscriber you are. The key itself never leaves the card.
In 5G
SNI
Server Name Indication
The TLS extension that tells the server which site is wanted, so one IP address can host many certificates. Historically sent in the clear.
In URL
SNonce
Supplicant Nonce
The random number the client contributes to the four-way handshake, paired with the access point's ANonce.
In Wi-Fi
SNR
Signal-to-Noise Ratio
How far the signal stands above the background noise, in dB. It, rather than raw signal strength, sets the speed a link can sustain.
In Wi-Fi
SPF
Sender Policy Framework
A DNS record listing which servers may send mail for a domain.
In URL
SPI
Serial Peripheral Interface
The simple bus connecting the processor to the flash chip holding the firmware — the first thing it reads after reset.
In Startup
SSD
Solid-State Drive
Storage with no moving parts. Fast enough that the boot delay you notice is almost never the disk any more.
In Startup
SSID
Service Set Identifier
The network name you pick from a list. It is not an address and it is not unique — anything can broadcast any name.
In Wi-Fi
SYN
Synchronise
The first packet of a TCP connection, answered by a SYN-ACK and then an ACK. That is the three-way handshake, and it costs a round trip before any data moves.
In URL
T
TCP
Transmission Control Protocol
The protocol that turns unreliable packets into an ordered, reliable stream, by numbering everything and resending what is not acknowledged.
TLS
Transport Layer Security
The encryption layer under HTTPS. It proves the server's identity with a certificate and then encrypts everything that follows. Formerly called SSL.
TPM
Trusted Platform Module
A separate chip that records what was loaded during boot and will only release the disk encryption key if those measurements match.
In Startup
TTFB
Time To First Byte
How long from sending the request to the first byte of the response arriving. It separates network and server delay from rendering delay.
In URL
TTL
Time To Live
How long a cached DNS answer may be reused before it must be looked up again. A low TTL is what makes a planned change take effect quickly.
In URL
TXT
Text record
A DNS record holding free-form text. In practice it carries verification strings and mail policies such as SPF and DKIM.
In URL
U
UDP
User Datagram Protocol
Send-and-hope: no connection, no ordering, no retransmission. Useful when a late packet is worth less than a lost one, and the base QUIC builds on.
In URL
UEFI
Unified Extensible Firmware Interface
The modern replacement for the BIOS. It can read filesystems, has drivers, a shell and its own variable storage, and it is what holds the boot order.
In Startup
URL
Uniform Resource Locator
A web address: the scheme, the host, the path and anything after it. It names both where a thing is and how to ask for it.
In URL
USB
Universal Serial Bus
The general-purpose connector for peripherals and, at boot time, one of the devices the firmware can be told to start from.
In Startup
V
VLAN
Virtual LAN
One physical network divided into several logical ones, so guest and corporate traffic can share the same cables and stay separate.
In Wi-Fi
VPN
Virtual Private Network
An encrypted tunnel carrying your traffic to somewhere else before it reaches the internet. It moves who can see your traffic; it does not remove the need for HTTPS.
W
Wi-Fi
Not an acronym and not short for anything — a trademark the Wi-Fi Alliance invented because "IEEE 802.11b Direct Sequence" was never going to sell.
In Wi-Fi
Wi-Fi 6E
Wi-Fi 6 Extended
Wi-Fi 6 given access to the new 6 GHz band: the same radio standard with far more clean spectrum to use.
In Wi-Fi
WPA2
Wi-Fi Protected Access 2
The long-standing Wi-Fi security standard, using a four-way handshake and CCMP encryption. Its handshake can be captured and guessed at offline.
In Wi-Fi
WPA3
Wi-Fi Protected Access 3
The current standard. It replaces the handshake with SAE, so capturing it no longer gives an attacker anything worth taking away.
In Wi-Fi